1 EX/P3-05. Experimental Investigation of Plasma Confinement in Reactor Relevant Conditions in TCV Plasmas with Dominant Electron Heating

Transcription

1 1 EX/P3- Exprimntal Invstigation of Plasma Confinmnt in Ractor Rlvant Conditions in TCV Plasmas with Dominant Elctron Hating N A Kirnva 1 R Bhn 2 G P Canal 2 S Coda 2 B P Duval 2 T P Goodman 2 B Labit 2 N A Mustafin 1 A N Karpushov 2 A Pochlon 2 O Sautr 2 M Silva 2 B Tal 3 and th TCV tam 1 Institut of Physics of Tokamaks National Rsarch Cntr Kurchatov Institut Kurchatov Sq. 1 Moscow Russia 2 Ecol Polytchniqu Fédéral d Lausann (EPFL) Cntr d Rchrchs n Physiqu ds Plasmas Association Euratom- Confédération Suiss CH-11 Lausann Switzrland 3 Institut for Particl and Nuclar Physics Wignr Rsarch Cntr for Physics Hungarian Acadmy of Scincs Association EURATOM P. O. Box 49 H-12 Budapst Hungary -mail contact of main author: kirnva@nfi.kia.ru Abstract. Rcnt xprimnts hav bn prformd on th TCV tokamak to invstigat confinmnt in lctronhatd dischargs in an attmpt to simulat ractor conditions with dominant lctron hating. For th first tim L-mod confinmnt proprtis hav bn analyzd as a function of th lctron hating powr profil width. Dischargs with on-axis pakd lctron-cyclotron hating powr profils with HWHM~ as wll as a scnario with off-axis hating pakd at ~.7 hav bn studid in th rang of hating powr.-2 MW at plasma dnsity m -3 =.1 n. Th confinmnt appars to b indpndnt of th powr dposition profil width providd a significant fraction is insid th q=1 radius. Confinmnt dgradation scals with th hating powr in agrmnt with ITER scaling prdiction for on-axis pakd powr dposition whras a strongr dgradation is obsrvd for off-axis hating E ~ P -.9 tot. Plasma bhavior has also bn invstigatd in dischargs with highr plasma dnsity. A limit dnsity clos to and vn xcding th Grnwald limit nlim.6 1.1n has bn achivd in ohmically hatd plasmas dpnding on th q 9 valu. 1. Introduction A crucial rcnt vnt in th fild of controlld nuclar fusion was th bginning of th construction of th Intrnational Thrmonuclar Exprimntal Ractor (ITER). On of th ky issus to b addrssd by this dvic is th dmonstration of a burning fusion plasma. Howvr many physics issus rmain opn and rquir additional xprimntal fforts in prsnt day machins to prpar for fficint ITER opration. Plasma hating in a thrmonuclar ractor will b carrid out mainly by alpha-particls lading to dominant lctron hating. Simulating such conditions in xisting tokamaks with dominant lctron hating is thrfor rlvant to th prparation of ractor scnarios. Th TCV tokamak is quippd with a vry flxibl lctron cyclotron hating systm consisting of 6 gyrotrons of frquncy 82.7 GHz for th injction of powr (. MW pr gyrotron) at th scond harmonic (X2) of th lctron cyclotron rsonanc (ECR) and 3 gyrotrons at 118 GHz th third harmonic (X3). Th bam launching gomtry can b st for ach X2 gyrotron sparatly allowing th hating powr to b distributd widly in a highly controlld way along th plasma minor radius. TCV is thrfor wll suitd for xprimnts aimd at studying th plasma bhavior in rgims with dominant lctron hating in ractor rlvant conditions. 2. Exprimntal conditions An xprimntal scnario with varying hating powr dposition profil width has bn dvlopd to invstigat th plasma rspons to dominant lctron hating. A wid rang of

2 2 EX/P3- dposition widths up to a valu approaching both th ohmic hating powr width and that stimatd in [1] for alpha hating in ITER in on of th stady stat scnarios dscribd in [2] (FIG. 1). Th ECR hating powr absorption has bn calculatd by th ray-tracing Toray- GA cod [3]. Ths xprimnts wr prformd in plasmas with modrat longation = both positiv and ngativ triangularitis q 9 =4 and n 2 1 m.1n [4]. In a furthr st of xprimnts th plasma dnsity was incrasd to th limit dnsity valu in ordr to invstigat confinmnt in th vicinity of th dnsity limit th dnsity limitation procss itslf and th possibl rol of ECRH in prvnting dnsity limit disruptions. For this purpos a combination of X2 and X3 hating was usd sinc th cntral dnsity xcdd th cutoff cut off 19-3 dnsity for th X2 frquncy ( n 4 1 m ). P hat ( )/P max c) d) ) FIG. 1 Hating powr profils normalisd to thir pak valus. Solid rd lin: ECRH hating powr profil; dash-dottd lin: ohmic hating powr profil; solid gry lin: alpha hating profil xpctd in ITER stady-stat scnario (copid with prmission from [1]). On-axis hating with 1/2 ~.12 ( 1/2 = HWHM of ECRH powr profil); on-axis hating with 1/2 ~.1; c) on-axis hating with 1/2 =.3; d) on-axis hating with 1/2 =.4;) far offaxis hating. Radius of q=1 surfac is shown by arrow for ach shot. In cas of far off-axis hating th shift of q=1 position during th powr incras is also shown. 3. Confinmnt dpndnc on hating powr profil width Th typical scnario undr study is shown in FIG. 2. Th hating powr was applid in 3 stps to invstigat th powr dpndnc of E in a singl shot. Th duration of ach powr stp. s was chosn to analyz th confinmnt proprtis in quasi-stady-stat conditions at ach powr stp. All dischargs discussd hr ar similar during th ohmic phas until th ECR hating is applid. A significant diffrnc is obsrvd aftrwards btwn shots with on-axis pakd hating and off-axis hating th formr lading to a much mor substantial incras in p (FIG 2c). This ffct cannot b xplaind by th diffrnc in th absorbd hating powr fraction P abs /P inj which is modst: P abs /P inj ~1% for on-axis hating irrspctiv of hating profil width whras P abs /P inj incrass from 86.4% to 91.4% and finally to 94.% at ach powr stp for far off-axis hating. Th dpndnc of th nrgy confinmnt tim on th hating powr profil width is prsntd in FIG. 3. Th nrgy confinmnt tim varis with nt hating powr in agrmnt with th prdiction of ITER L-mod scaling [] in all dischargs with on-axis paking of ECR powr absorption indpndntly of th hating profil width whras it dcrass mor rapidly in th cas of far off-axis hating scaling as E ~ P tot -.9. FIG. 4 shows th ratio of

3 n I p ka 3 EX/P3- ECRH powr absorbd insid th q=1 surfac P 1 to th total absorbd ECRH powr. A significant fraction of th powr is absorbd insid th q=1 rgion in all cass with on-axis ECRH paking whras in th cas of far off-axis absorption all th ECRH powr is absorbd outsid th q=1 rgion (FIG.1). p P ECRH MW 1 19 m on-axis.2 off-axis c) tim s FIG. 2 Typical scnario of th xprimnt for all shots prsntd in FIG. 1: plasma currnt and ECR hating powr which ar th sam by dsign for all shots undr discussion; lin-avragd dnsity; c) p. An additional st of xprimnts was prformd to vrify th consistncy of th trnds at lowr plasma currnt (I p =216 ka q 9 =4.8 P 1 /P ECRH ~.3) and at ngativ triangularity ( = -.43 and P I /P ECRH ~.8). Dischargs display th sam trnd in th nrgy confinmnt dpndnc on th hating powr: th sam confinmnt dgradation has bn obsrvd in ach cas in agrmnt with th ITER-L mod scaling E ~P tot All th rsults prsntd abov dmonstrat that th confinmnt proprtis do not dpnd on th width of hating powr profil and suggst that an important rol may b playd by th localisation of th hating powr with rspct to th q=1 surfac. 4. Rol of th absorption position rlativ to q=1 surfac In ordr to invstigat th ffct of th absorption position rlativ to th q=1 surfac an xprimnt was prformd with only on X2 gyrotron whos absorption position was movd by mans of a strabl mirror from far outsid th q=1 position (dtrmind through th sawtooth invrsion radius) to th magntic axis for both positiv and ngativ plasma triangularity (othr plasma paramtrs having bn fixd). Th rsult is prsntd in FIG. showing th dpndnc of th lctron nrgy confinmnt tim on th ECRH absorption position. Hr th lctron confinmnt tim is normalizd to th global Rbut-Lallia-Watkins scaling RLW [6] which is appropriat for L-mod plasmas with a hot lctron componnt. Whil th absorption layr is far outsid th q=1 surfac th lctron nrgy confinmnt is clos to that prdictd by RLW scaling for positiv triangularity and is noticably highr for th ngativ triangularity cas. Howvr th nrgy confinmnt tim incrass markdly whn th absorption layr passs across th q=1 surfac (indicatd by arrows in FIG. ). This rsult confirms th hypothsis that th confinmnt 2

4 4 EX/P3- proprtis dpnd strongly on th localization of additional hating with rspct to th q=1 surfac. Not hr that no pronouncd Intrnal Transport Barrir has bn obsrvd at th q=1 position. In all cass dscribd abov dnsity and tmpratur profils (both ion and lctron) ar smooth [4]. E ms ~P -.73 tot ~P -.9 tot P 1 /P ECRH P tot MW FIG. 3 Dpndnc of E on hating powr: circls: shots with on-axis hating localization irrspctiv of th hating profil width; triangls: far off-axis hating. Th solid lin corrsponds to a E ~P -.73 tot dpndnc and th dashd lin to a E ~P -.9 tot dpndnc. Both curvs ar matchd to th xprimntally masurd valu of E in th ohmic phas of th discharg /2 FIG. 4 Th fraction of EC hating powr dpositd insid th q=1 surfac for diffrnt hating powr profils obtaind from ray tracing calculations by th TORAY-GA cod. 2. ~ -. ~ +.3 E / RLW abs FIG.. Dpndnc of th ratio E / RLW on th man EC powr absorption position (dfind as th radius insid which half of th powr is absorbd) for TCV dischargs with ngativ (circls) and positiv (triangls) triangularity. Th q=1 radius is indicatd by arrows solid lin for < cas and dash-dottd lin for >. E : lctron nrgy confinmnt tim; RLW : lctron nrgy confinmnt tim givn by th Rbut-Lallia-Watkins scaling; abs : th position insid which half of th EC powr is absorbd.

5 I p ka EX/P3-. High dnsity xprimnts in TCV Th xprimnts dscribd abov wr prformd in plasmas with modrat dnsity n 2 1 m.1n whr anomalous transport losss in TCV ar attributd mainly to long scal lngth lctron turbulnc i.. Trappd Elctron Mods (TEM) [7]. Howvr a dnsity incras can lad to th xcitation of diffrnt instabilitis that could chang th confinmnt proprtis and dpndncs. Hnc an xtnsion of th dnsity rang is dsirabl. As a first stp a basic invstigation of th limit dnsity valu and of th dnsity limitation procss was prformd in conjunction with an analysis of thir dpndnc on ECRH. For this purpos th discharg paramtrs wr optimizd taking into account th cutoff dnsitis n m for X2 and n m for cut off 19-3 cut off 2-3 X3..1. Limit dnsity in Ohmic dischargs Ths xprimnts wr prformd using dnsity ramps in limitd plasmas with = =1.3 I p =2 ka and q 9 ~3.7; th Grnwald dnsity is n m. A fw dischargs at I p =11 ka and q 9 ~7 wr includd to chck th possibl dpndnc of maximal dnsity valu on plasma currnt. Th dnsity was controlld by a fdback control systm. Th typical xprimntal scnario is prsntd in FIG n 1 19 m tim s FIG. 6. Typical scnario of high dnsity xprimnts in TCV (Shot 4181). Tracs of plasma currnt ( and lin avragd plasma dnsity masurd along th cntral chord (. Th plasma currnt in a similar discharg but without th disruption is shown by th dash-dottd lin in plot (. A maximal valu of plasma dnsity n m.63n was achivd in th rgim with I p =2 ka whras a valu clos to th Grnwald limit dnsity was rachd in th low currnt rgim: n.6 1 m n. Th approach to th limit dnsity in ths TCV xprimnts had th pculiar fatur of bing accompanid by th disapparanc of sawtooth oscillations (Fig. 7). Normal sawtooth oscillations with th priod Ts ms ar sn until th dnsity is ~2% blow its limit valu (n lim ) at which point th sawtooth priod starts incrasing. Th sawtooth-fr phas starts at n.9n. At som point an m=1/n=1 mod dvlops and about ms latr a giant lim sawtooth-lik crash at n nlim (Fig. 7) is obsrvd. This crash triggrs an m=2/n=1 mod dvloping similarly to that dscribd in [8] causing a significant dcras of th plasma

6 T () kv n () P rad () I SXR arb.un. n 6 EX/P3- stord nrgy. This mod is stabilizd within ~1 ms. Th rst of th discharg is a squnc of sawtooth-lik crashs with a dcras in plasma dnsity incrasd gas-puffing and as a consqunc additional plasma cooling until th trmination of th discharg (about ms aftr n nlim has bn rachd). In th scnarios invstigatd hr th m=2/n=1 mod dos not appar to play a primary rol in th attainmnt of th dnsity limit. Morovr a possibl MARFE formation at th dnsity limit has not bn vidncd but cannot b xcludd. Furthr xprimnts ar rquird to clarify our prsnt undrstanding m m -3 MW/m tim s FIG. 7. TCV shot ohmic shot with a dnsity limit disruption. Tracs of lin avragd plasma dnsity ( SXR mission masurd along th cntral chord ( radiatd powr from th plasma cntr valuatd from foil bolomtrs (c) cntral plasma dnsity invrtd from FIR masurmnts (d) and cntral lctron tmpratur masurd by XR spctromtr (). Th start of th sawtooth fr phas and th first giant sawtooth crash ar markd by arrows. Th sawtooth disapparanc in ths shots is prcdd by a paking of th dnsity profil in th cntral rgion (insid q=1). This is accompanid by a modification of th currnt profil (obsrvd in th fact that th sawtooth invrsion radius is mor and mor localizd towards th cntr as th dnsity paks). Onc th sawtooth activity is stabilizd th radiation losss pak at th plasma cor as a rsult of continud cntral dnsity incras and impurity accumulation (FIG. 7 c)-)). This cools th plasma as obsrvd in th flattning of th tmpratur profil during th sawtooth-fr phas. Ths currnt profil dynamics hav bn modlld using th ASTRA transport cod [9]. Th lctron tmpratur and dnsity profils ar masurd by Thomson scattring z ff is assumd to b =2. Th ion tmpratur is calculatd using th noclassical ion thrmal conductivity with th plasma conductivity and bootstrap currnt calculatd from th noclassical formula proposd in [1]. Kadomtsv s sawtooth modl [11] has bn usd to tak into account th profil modification du to sawtooth crashs. Th rsulting q(r) profils at two tim instants during th normal sawtooth phas and in th sawtooth fr priod - ar prsntd in FIG. 8. This shows that th sawtooth fr priod is not du to th disapparanc of th q=1 surfac c) d) )

7 q 7 EX/P3- I SXR arb.un. n rathr it is associatd with a flattning of th q(r) profil and a corrsponding dcras of th magntic shar in th vicinity of q=1. Th prsistnc of th q=1 surfac is consistnt with th dvlopmnt of an m=1/n=1 mod as dtctd by MHD probs ~ ms bfor th giant sawtooth-lik crash. 4 t 1 t 2 1. OH X3 3 P hat MW/m vol FIG. 8. q(r) profils calculatd by ASTRA cod for shot prsntd in FIG. 7: t 1 =1.4 s t 2 =1.6 s vol FIG. 9. Hating powr distributions. Solid lin: X3 powr profil calculatd by TORAY-GA cod; dashdottd lin: ohmic powr distribution calculatd with ASTRA cod..2. Limit dnsity with additional ECRH In th light of th prvious obsrvations ECR hating was applid to prvnt sawtooth stabilisation at high dnsity. Du to th rlativly high valu of achivabl dnsity ithr X3 or a combination of X2 (during.-.9 s) and X3 was usd. A typical profil of th absorbd ECRH powr is prsntd in FIG. 9 in comparison with th Ohmic powr for th discharg illustratd by Figur 1. Th total absorbd X3 powr (stimatd from TORAY-GA) is ~ 3- kw which is a small fraction of th ohmic powr (28 kw). Th absorption layr is clos to and slightly insid th q=1 surfac (s FIGs. 8 and 9). Th main ffct of th X3 hating is obviously sn in th sawtooth dynamics (FIG. 1). Th sawtooth-fr phas is dlayd lading to an incras in th maximal achivabl dnsity. Th subsqunt disapparanc of sawtth and th vntual dnsity limit disruption may b a rsult of th rduction in X3 absorption du to th furthr dnsity incras m X3 OH tim s FIG. 1. Comparison of plasma bhavior in an ohmic shot and a shot with X3 hating. Tracs of lin avragd plasma dnsity ( and SXR mission masurd along th cntral chord (. Dnsity valus typical of th sawtooth-fr phas for both typs of dischargs ar shown by gry dots.

8 8 EX/P3-6. Conclusions Th influnc of th hating powr profil width on th confinmnt proprtis at modrat plasma dnsity ( n 2 1 m.1n ) has bn invstigatd in TCV in dischargs with dominant lctron hating. It has bn shown that th nrgy confinmnt tim dos not dpnd on th hating profil width in th rang 1/2 ~.1-.4 in cas of on-axis paking of absorption. A confinmnt dgradation with hating powr is obsrvd in this scnario with a dpndnc E ~ P tot Shifting th ntir hating zon off-axis on th othr hand lads to strongr confinmnt dgradation E ~P tot -.9. A dynamic xprimnt moving th absorption layr from outsid to insid th q=1 rgion has dmonstratd a clar confinmnt improvmnt whn th absorption layr passs through th q=1 zon implying that th location of th hating powr rlativ to th q=1 surfac position is th dominant playr in dtrmining th confinmnt. Th dnsity limitation procss has bn invstigatd in TCV plasmas in ohmic and ECR hatd dischargs for diffrnt plasma currnt lvls. A dnsity limit clos to th Grnwald limit was rachd at lowr currnt I p =11 ka whras at I p =2 ka th limit is lowr ( nlim.63n ). Th dnsity limit was found to b dpndnt on th plasma currnt distribution in th vicinity of th q=1 surfac with a tmporary stabilization of sawtth prcding th disruption. Accordingly sawtooth dstabilization by ECR hating has bn shown to incras th dnsity limit in this scnario. This work was carrid out in th framwork of th "Scintific and Tchnological Coopration Programm Switzrland Russia Utilization of Spcific Infrastructur Projcts". This work was supportd in part by th Swiss National Scinc Foundation. Russian participants of th work wr supportd by th Russian Ministry of Education and Scinc (Contract No ) and Rosatom (Contract No. H.4x ). [1] KUKUSHKIN A.B. t al Proc. 23rd IAEA Fusion Enrgy Conf. Dajon South Kora 21 ITR/P1-34 [2] POLEVOI A.R. t al. Proc. 37th EPS Conf. on Plas.Phys. Dublin Irland 21 P2.187 [3] MATSUDA K. IEEE Transactions on Plasma Scinc 17 (1989) 6 [4] KIRNEVA N. A. t al Plasma Physics Control. Fusion 4 (212) 111 [] ITER Physics Basis Nuclar Fusion 39 (1999) 2137 [6] REBUT P. H. t al IAEA Vinna Proc. of th 12th Intrnational Confrnc on Plasma Physics and Controlld Nuclar Fusion Rsarch Nic 1988 Intrnational Atomic Enrgy Agncy Vol. 2 p. 191 [7] MARINONI A. t al Plasma Phys. Control. Fusion 1 (29) 16 [8] CANAL G.P. t al Proc. 39th EPS Conf. and 16th Int. Congrss on Plasma Physics Stockholm Swdn 2-6 July 212 P.76 [9] PEREVERZEV G. V. YUSHMANOV P. N. ASTRA - Automatd Systm for TRansport Analysis IPP /98 Fbruary 22 [1] SAUTER O. t al Phys. Plasmas 6 (1999) 2834; ibidm 7 (2) 3122 [11] KADOMTSEV B.B. Sov. J. Plasma Phys. 1 (1976) 389